2D Materials: Crystal Growth for Future Device Structures

Abstract

The isolation of graphene, now over decade ago, has given rise to the revitalization of an old full set of materials, two-dimensional materials (2DM), which have exceptional electrical, chemical and physical properties. Some of the materials under investigation in addition to graphene are hexagonal boron nitride (h-BN), semiconducting, metallic, and superconducting, transition metal dichalcogenides (TMD) with a general chemical formula, MX₂ where M is for example equal to Mo, W, Ta, Nb, Zr, Ti, and X = S, Se and Te, and others. While graphene is a material with many exceptional properties and h-BN is an excellent 2D insulator, TMD materials provide what neither graphene nor h-BN can, bandgap engineering that, in principle, can be used to create new devices that cannot be fabricated with h-BN and graphene alone. There is hope to that 2DM can be integrated to fabricate numerous device types for many applications ranging from inkjet-printed circuits, photonic applications, flexible electronics, and high performance electronics. However, in order to fully realize the benefits of these materials, the community will have to work together to define the device structures, device integration schemes, and materials growth processes and requirements. 

A number of deposition techniques have been used to prepare large area graphene, such as growth on SiC through the evaporation of Si at high temperatures, precipitation of carbon from metals, and chemical vapor deposition on Cu. Direct growth of good quality graphene on dielectrics/semiconductors other than SiC with reasonable properties has only been reported recently on Ge. The preparation of large area h-BN is also in great demand and processes are being developed to achieve this on both metals and dielectrics. Transition metal dichalcogenides present altogether different opportunities and difficulties in the preparation of low defect density large area single crystals.  Vapor transport, chemical vapor deposition (CVD), and molecular beam epitaxy (MBE) are being developed to produce these materials for initial studies of materials physics device fabrication. In addition, there is some effort in performing simulations to guide growth for both CVD and MBE growers.  Therefore, there is an opportunity here to have the crystal growers and the modeling community to collaborate with each other to develop high quality materials and processes.

A number of devices structures are currently under evaluation to take advantage of the basic properties of graphene, bi-layer graphene, h-BN and TMDs. Some of the devices are based on tunneling phenomena while others are based on excitonic phenomena. In this presentation I will present the state of the art results of graphene, h-BN, and a few TMD materials and their prospects for future electronic device applications.